Often when analyzing process streams using mass spectroscopy or other analysis techniques, the sampling system is the most important part of generating meaningful data. A properly functioning sampling system must withdraw a small amount of material from a process stream, and deliver it to an analysis instrument in a substantially intact condition.
In some process streams, for example acrylonitrile streams, there are small amounts of materials that boil at very high temperatures. Those components of the sample may condense out of the streams and plug the sampling lines. One frequently-used solution to that problem is to heat the sample stream to extremely high temperatures (180 degrees C.) to keep those components in the gas phase. That technique requires large amounts of energy and considerable expense. In some cases, the temperatures required are so high they cannot be reached using available equipment. Further, there may be materials present in the stream that decompose at such high temperatures, making such a solution impractical.
One example of a mass spectrometry sampling system that is heated to avoid condensation is shown in U.S. Pat. No. 3,944,824 to Chagney et al. That system samples gaseous compounds from a main process line, and includes a sampling line that is traced with steam to assure that no condensates reach the mass spectrometer.
U.S. Pat. No. 6,670,608 to Taylor et al. discloses a gas sampling system for sampling hazardous process gasses for analysis in an instrument that is remote from the process line. The technique uses a small-diameter capillary to transport a sample from the process stream to a mass spectrometer chamber. The capillary tube is open at one end to the high vacuum environment of the mass spectrometer chamber, and at the other end to the process stream. The capillary tube diameter is chosen based on the pressures in the sample stream and the mass spectrometer. A capillary tube heater is provided to maintain the sample in the capillary tube above boiling point.
It is known to control the temperature and pressure of a desolvation chamber when preparing a liquid sample for introduction into a mass spectrometer chamber. For example, in U.S. Pat. No. 4,403,147 to Melera et al., a jet stream of liquid droplets is sprayed through a probe into a low pressure, high temperature chamber for evaporation before entering the mass spectroscopy chamber.
There is therefore presently a need to provide a method and apparatus for sampling a process stream containing high-boiling-point gaseous components for analysis in a test instrument such as a mass spectrometer. Particularly, there is a need for a technique that can prevent condensation of the high-boiling-point components in the sampling system without excessively heating the sample. To the inventors' knowledge, no such technique is currently available.